1. Field of the Invention
The present invention relates to a method for low frequency cancellation of noise in magneto-resistive mixed sensors.
The invention more specifically relates to a method for cancellation of low frequency noise in a magneto-resistive mixed sensor comprising at least a superconducting loop with at least one constriction and at least one magneto-resistive element.
Low frequency noise refers to the resistance noise of the magneto-resistive element created by low frequency fluctuations. This kind of noise appears in all physical systems and increases as the size of the system decreases. If f is the frequency of the low frequency fluctuations, the power density of the noise decreases as 1/fα where α is of the order of 1.
2. Description of the Related Art
The principle of mixed sensors which associate at least one superconducting loop and one magneto-resistive element for low frequency and RF applications is described in documents WO 2004/068152 A1 and WO 2004/068158 A1. The principle of the mixed sensors is also described in the publication Pannetier M., Fermon C., Le Goff G., et al. SCIENCE 304 (5677): 1648-1650 Jun. 11, 2004.
Each superconducting loop contains at least one constriction. When a magnetic field is applied on the said superconducting loop, a super-current is created in the loop. The super-current flows through the constriction and locally the current density is high. The at least one magneto-resistive element is placed on top or bottom of the constriction and senses the magnetic field created by the super-current. The ratio between the applied field and the magnetic field sensed by the magneto-resistive element can be several thousands for a cm2 superconducting loop and micron size constriction. The low frequency noise of a mixed sensor is due to the low frequency resistance noise of the magneto-resistive element.
Low frequency noise levels of magneto-resistive elements have been extensively studied in the published literature. For example, Giant magneto-resistance (GMR) low frequency noise is discussed in Raquet B, Viret M, Costes M, et al., JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS 258: 119-124 Sp. Iss. SI MARCH 2003. Noise in Tunnel magneto-resistive sensors (TMR) is discussed in L. Jiang et al, Phys. Rev. B 69, 2004 p 54407.
If the magnetic part of the low frequency noise can be cancelled by a proper design of the sensor (see Pannetier M., Fermon C., Le Goff G., et al. JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS 290: 1158-1160 Part 2 Sp. Iss. SI APRIL 2005), the non magnetic part has at present not be cancelled.
Document WO 2004/068152 A1 describes the use of the saturating value of the magneto-resistive element as a reference to correct temperature and ageing variations. However, this technique is not suppressing the low frequency noise of the magneto-resistive element.
Document PCT/2006EP/002599) describes the use of modulation of the sensing current for moving the working frequency of the detection outside the field detection frequency. This technique can be used to work at a frequency higher than the low frequency noise but if it suppresses the low frequency noise coming from the preamplification chain, it does not suppress the noise coming from the resistance fluctuations.
Fluxgates use a field modulation technique based on their non linear response to suppress the low frequency noise (see Magnes W, Pierce D, Valavanoglou A, Means J, Baumjohann W, Russell C T, Schwingenschuh K, Graber G, Source: MEASUREMENT SCIENCE & TECHNOLOGY 14 (7): 1003-1012 JULY 2003). Use of harmonics with pulsed currents has also been proposed (see Kubik J, Ripka P, SENSORS AND ACTUATORS A-PHYSICAL 132 (1): 236-240 Sp. Iss. SI, Nov. 8, 2006).
These approaches cannot be applied to mixed sensors due to their linear response in the working field range.
This technique has also been tested on mixed sensors (Document WO 2004/068152 A1) but its efficiency, which is proportional to the second derivative of the resistance variation, is not sufficiently competitive due to the low non linearity of the magneto-resistive sensors.
More generally, for all types of magnetic sensors, it is known that a cancellation of the low frequency noise of a detection system can be done by a modulation of the source magnetic field at a sufficiently high frequency. Then the measurement of the source field is done around the frequency modulation and the low frequency noise of the detection system does not interfere with the measurement. A large number of publications and patents are applying that technique.
However, that technique cannot be applied when the field source cannot be modulated. This is the case of for example, the detection of magnetic fields created by the human body.
More specifically, in Hall effect based magnetic sensors, a high frequency switching of the current input and voltage output allows suppressing the low frequency noise. A description of such a method can be found in P. Munter, A low-offset spinning-current Hall plate, Sens. Actuators A: Phys. A22 (1990) (1-3), pp. 743-746 However, that technique is very specific to Hall sensors due to the vectorial nature of the Hall resistance (the measured voltage is perpendicular to the applied current direction) and cannot be applied to magneto-resistive sensors because the measured voltage is along the applied current direction.